1. Field of the Invention
Exemplary aspects of the present invention generally relate to an electrophotographic image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, that forms an image using toner.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a transfer medium such as a sheet of paper according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to make the electrostatic latent image visible as a toner image; a transfer device transfers the toner image directly from the image bearing member onto a transfer medium or transfers the toner image from the image bearing member onto a transfer medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the transfer medium; finally, a fixing device applies heat and pressure to the transfer medium bearing the unfixed toner image to fix the unfixed toner image on the transfer medium, thus forming the image on the transfer medium.
Typically, a guide member disposed at the entrance of the fixing device guides a transfer medium, onto which the toner image is transferred, to a fixing nip defined by and between a fixing member and a pressing member of the fixing device. Heat and pressure are applied to the transfer medium, thereby fixing the toner image in place on the transfer medium. The guide member needs to convey the transfer medium reliably from the transfer device to the fixing device. Reliable conveyance of the transfer medium depends largely on the shape and electrical characteristics of the surface of the guide member. For example, if friction between the guide member and the transfer medium is significant, an electrostatic charge builds up on the guide member and stored charge of the transfer medium fluctuates, causing unfixed toner on the transfer medium to scatter uncontrollably. In particular, toner tends to scatter more easily with a transfer medium having relatively high resistance in a low-humidity environment. When such a transfer medium is guided by the guide member, friction between the leading edge of the guide member and the transfer medium becomes significant, so that the leading edge of the guide member is frictionally charged too much, increasing the electric potential and thus strengthening the electric field generated between the guide member and the fixing roller, which in turn generates an electrical discharge (leak) therebetween when the transfer medium enters the fixing nip. As a result, the unfixed toner in the toner image on the transfer medium scatters uncontrollably. Typically, the guide member tends to be frictionally charged easily after a certain period of operation, for example, after processing approximately 100 transfer media sheets.
Furthermore, when the toner image includes a significant amount of the colors red, green, blue, and the like consisting of two toners among magenta, cyan, and yellow, any difference between the electric potential of the transfer medium and the electric potential of the guide member causes the toner in the unfixed toner image to scatter uncontrollably. This phenomenon is referred to as two-toner scattering. Because the guide member is formed of electrically grounded sheet metal, the surface potential of the guide member is 0 V. Thus, there is a difference in the electric potential between the guide member and the transfer medium, causing the toner to scatter.
Moreover, in a case in which absorption or a contact force of the guide member relative to the transfer medium is strong, an image defect occurs easily at the guide member.
In view of the above, the guide member is coated with a film consisting of combination of metallic and ceramic particles, or a fluorocarbon resin film whose volume resistance is adjusted. In this configuration, the electrical characteristics of the surface of the guide member are adjusted so that the transfer medium can be transported reliably in both high- and low-humidity environments in which the electrical characteristics normally fluctuate.
Although advantageous, because the guide member is formed of molded resin coated with fluorocarbon resin, an electrical charge accumulates at the guide member over time (after processing approximately 100 transfer media sheets), causing the problems described above.
In another approach, the surface of the guide member is provided with a releasable/slidable layer laminated on an elastic sheet to reduce friction between the transfer medium and the guide member when the transfer medium comes into contact with the guide member, hence preventing deterioration of imaging quality. However, in order to produce the guide member coated with material to adjust the electrical characteristics of the surface of the guide member to reduce toner scattering caused by frictional charging, the known approaches described above require a number of manufacturing steps including initial processing of a base material, quenching, heating/drying, cooling, coating, and calcining. If the coating includes multiple layers, coating time, speed, angle, and so forth depend on types of coating compositions, thus complicating manufacture to achieve desired electrical and mechanical characteristics.